a. Field of the Invention
The present invention relates generally to methods and apparatus for construction of earthfill dams and, more particularly, to a method and apparatus for constructing the internal filter zones of earthfill dams by depositing and working the fine and coarse materials that form the filter zones on a simultaneous basis rather than separately.
b. Related Art
In the past, earthfill dams were constructed as a more or less homogenous mass of earth, excavated from a suitable borrow area near the construction site. Modern earthfill dams are rather more complex structures having a number of features that have been adopted both to increase the longevity of the dam and to prevent the failure, sometimes catastrophic, to which earlier earthfill dams were sometimes subject.
One particular feature of modern earthfill dams is the inclusion of filter zones, typically on both the upstream and downstream sides of the core. For example, FIG. 13 provides a simplified view of an earthfill dam 10 having a core 12 flanked by upstream and downstream filter zones 14, 16. Seepage flow passes through the filter zones and is collected by drains so as to be conveyed out of and away from the dam structure, e.g. through a blanket drain 18 as shown in FIG. 13.
The coarse and fine layers 20, 21 of the filter zones cooperate to prevent migration of soil particles while still providing for passage of the seepage flow. In this manner, the seepage is collected and permitted to drain freely, but piping (removal) of soil/fill particles from the dam (which might cause serious damage or even complete failure) is prevented. The coarse and fine filter materials may be, for example, crushed rock and sand, and are covered by additional fill that forms the main berm 24 of the dam.
In order to effectively intercept seepage flow, the filter zones extend upwardly over substantially the full height of the dam, generally vertically or at a steep angle as shown in FIG. 13. Consequently, the filter zones cannot be made by simply placing two layers sequentially one on top of the other, but instead must be built up vertically next to one another, usually simultaneous with the construction of other parts of the dam, e.g., the berm/shell. Conventionally, this has been done by first depositing a load of one of the materials (e.g., the coarse material) in a narrow row and compacting it, then bringing in and depositing a separate load of the second (e.g., fine) material and repeating the process alongside the first row, with the sequence being repeated over and over as the dam is built up to its full height.
The conventional method of constructing filter zones is thus laborious and inefficient. Furthermore, it is difficult to deposit, arrange and compact the two separate materials with the required degree of accuracy. In most instances, the two layers of the filter zones are only a few feet (e.g., 3-8 feet) thick, so that working them requires a degree of accuracy and finesse that is difficult to achieve with conventional earth-moving machinery. Moreover, depositing and working the two layers of material next to one another on an alternating basis tends to cause an undesirable degree of mixing to occur at the interface where the materials meet.
Consequently, the conventional methods and apparatus used to construct the filter zones are laborious and add significantly to the cost of constructing a typical earthfill dam. Moreover, the conventional methods and techniques are difficult and prone to errors that can, in some cases, be extremely expensive to rectify.
Accordingly, there exists a need for a method and apparatus for constructing the filter zones of earthfill dams that does not require the adjoining layers of coarse and fine fill material to be deposited and worked on a separate basis. Furthermore, there exists a need for such method and apparatus that enables the filter zones to be constructing rapidly and efficiently with reduced likelihood of error. Still further, there exists a need for such a method and apparatus that uses economical systems and that is compatible with other forms of earthmoving equipment that are employed on earthfill dam construction projects.